1. Technical Field
The present invention relates to an ultrasonic speaker and a projector incorporating the ultrasonic speaker, and more particularly, to an ultrasonic speaker capable of utilizing all the sound waves emitted from both sound wave output surfaces of a push-pull electrostatic ultrasonic transducer as a sound source and a projector.
2. Related Art
Recently, applications related to a combination of speakers having a parametric effect that uses the non-linearity of air with respect to ultrasonic waves and a reflection plate that reflects audible sound waves have been filed.
For example, a structure in which a reflection plate and an ultrasonic transducer array are incorporated into a speaker box is disclosed as an invention. According to the structure of this invention, however, sound wave output surfaces of the ultrasonic transducer array are not spaced at regular intervals with respect to the reflection surface, which poses a problem that sound pressures of reflected sounds are not uniform (for example, see JP-A-61-123389).
An invention to solve this problem has been also disclosed (for example, see Japanese Patent No. 2786531). This invention solves the problem by forming an ultrasonic transducer array on a concave surface of a parabolic substrate having an opening at the center, and providing a reflection plate for audible sound waves in the vicinity of the center point of the radius of curvature of the substrate so that secondary waves (audible sound waves) having a strong directivity are reflected on the reflection plate to be emitted through the hole (opening) provided at the center of the parabolic substrate.
However, these two methods relate to a transducer having a single sound wave output surface, and sound waves emitted to the backside have not been used effectively in a push-pull electrostatic transducer of such a structure that sound waves are output in directions toward both surfaces of the transducer.
FIGS. 10A through 10C are views used to describe the driving concept of a push-pull electrostatic ultrasonic transducer. The push-pull electrostatic ultrasonic transducer is provided with a pair of a counter fixed electrode portion 51 and a counter fixed electrode portion 52 each opposing a vibration film 41. A plus (+) DC bias is applied to the vibration film 41 from a DC bias power supply, whereas an alternating signal is applied between the counter fixed electrode portion 51 and the counter fixed electrode portion 52.
FIG. 10A is a view showing an amplitude state of the vibration film 41 when an alternating signal is zero (0). In this state, the vibration film 41 is present at a neutral position (at the middle between the counter fixed electrode portion 51 and the counter fixed electrode portion 52). FIG. 10B is a view showing an amplitude state of the vibration film 41 when a plus (+) voltage of an alternating signal is applied to the counter fixed electrode portion 51 while a minus (−) voltage of the alternative signal is applied to the counter fixed electrode portion 52. In this state, the center of the vibration film 41 is attracted in a direction toward the counter fixed electrode portion 52 by an electrostatic force (attraction force) induced between the vibration film 41 and the counter fixed electrode portion 52 and an electrostatic force (repulsive force) induced between the vibration film 41 and the counter fixed electrode portion 51.
FIG. 10C is a view showing an amplitude state of the vibration film 41 when a minus (−) voltage of an alternating signal is applied to the counter fixed electrode portion 51 while a plus (+) voltage of the alternating signal is applied to the counter fixed electrode portion 52. In this state, the center of the vibration film 41 is attracted in a direction toward the counter fixed electrode portion 51 by an electrostatic force (attraction force) induced between the vibration film 41 and the counter fixed electrode portion 51 and an electrostatic force (repulsive force) induced between the vibration film 41 and the counter fixed electrode portion 52. In this manner, the vibration film 41 vibrates in response to an alternating signal and thereby generates sound waves. The sound waves generated by the vibration film 41 are emitted in directions toward both surfaces, respectively, on the sides of the counter fixed electrode portion 51 and the counter fixed electrode portion 52.
When the push-pull ultrasonic transducer of a structure wherein sound waves are output in directions toward both surfaces in this manner is used, either sound waves outputted from both surfaces of the fixed electrode 50 are emitted (leaked) intact as is shown in FIG. 11A, or sound waves outputted from the counter fixed electrode portion 52 are attenuated by an absorber 90 as is shown in FIG. 11B. Therefore, the ultrasonic speaker is not configured to fully use all the sound waves outputted from the ultrasonic transducer.
In addition, as is shown in FIG. 12A, an ultrasonic speaker in the related art is of a circular shape, and it is incorporated into the projector in a standing posture with its sound wave output surface facing toward the front. Hence, when a reflection plate 60a is provided behind the circular fixed electrode 50, the reflection plate 60a needs to have a diameter twice as large as the diameter of the fixed electrode 50, which makes it difficult to easily fold the reflection plate 60a. This poses a problem in that the projector is increased in size or outputs of the ultrasonic speakers are insufficient (FIG. 12B shows a mode when the ultrasonic transducers shown in FIG. 12A are incorporated into the projector).